This appears to be a standard dagger, but its hilt holds a hidden store of poison. Any critical hit injects poison into the target. A creature injected with the poison must succeed on a DC 12 Constitution saving throw or take 2d4 poison damage. The dagger holds up to six doses of poison. If the hilt contains fewer than six doses, the owner can pour more in up to the maximum.
The statistics above indicate use of “Poison, Basic” from the Player’s Handbook. The increased damage and higher saving throw target reflect the enhanced delivery system, as only a grievously precise wound will trigger the delivery and more poison is able to enter the bloodstream. Each “dose” mentioned above is the equivalent volume of a vial of basic poison, so completely refilling the dagger would normally cost around 600gp for 6 vials of poison.
At the time you got the dagger, it was completely full of basic poison.